1. Field of the Invention
The present invention relates to an image processing apparatus which executes quantization of image data into binary data or multi-level data, particularly an image processing apparatus which executes intermediate shade (or gradation) processing of input image data.
2. Related Background Art
For the semi-intermediate (or pseudo gradation) processing system of the image processing apparatus such as facsimile or digital copying machine, others have so far proposed error a diffusion method, an average density approximation method, etc.
The error diffusion method is a method to binarize the multi-level image data of an objective pixel (to convert it into darkest level or shortest level), give specific weight to the difference between the binarized level and multi-level image data prior to binarization and add it to the data of the pixel near the objective pixel as disclosed in the literature of R. Floyd & L. Steinberg, entitled "An adaptive algorithm for spatial gray scale", SID 75 DIGEST, pp. 36-37.
The average density, approximation method is a method, as described in Japanese Unexamined Patent Publication (Kokai) No. 57-104369, wherein the weighted means value of the respective weights of the objective pixel binarized into black and white and that of the neighbouring pixel are obtained using the already binarized binary data of the pixel near the objective pixel and thereby the image data of the objective pixel are binarized using the average of the two mean values as the threshold.
Since the aforesaid error diffusion method is used to correct the error between the input image data and the output image data, it is possible to maintain the input image and the density of the output image processing unit and produce the image with excellent resolution and shade characteristics.
However, during the correction of the error between input image data and output image data, many two dimensional calculations must be made and the magnitude of processing volume is so large that the composition of hardware inevitably becomes complicated.
On the other hand, according to the average density approximation method, calculation is made using binary data after binarization and consequently the composition of hardware may be simplified and the processing can be made at a high speed since the processing volume is extremely small.
However, this method has a drawback since binarization is conducted simply through approximation of an objective pixel by the average value of the range which contains the objective pixel, number of shades is limited and a specific low frequency texture is generated to the image which has the gradually changing density and picture quality deteriorates.
Besides, the drawback common to these systems is that when the manuscript image is divided into more than two regions, discontinuity occurs between the data of the divided images and thus picture quality deteriorates.
In other words, the error diffusion method has such a drawback that at the point near the starting point of binarization of a divided image, the error for correction is not transmitted and it tends to become the so-called binarization processing by fixed threshold.
In view of the above, the applicant of the present invention has applied for a patent in U.S. patent application Ser. No. 203,880, now issued as U.S. Pat. No. 4,958,236, for the technology to provide continuity to the plural number of regions at the binarization by the error diffusion method.
However, the average density approximation method has such a drawback that at the calculation of average density, the already binarized data are not repeated and consequently the expression of intermediate shade becomes discontinuous at the connecting part between the divided images after binarization processing and thus picture quality deteriorates.